Photovoltaic power plant

ABSTRACT

Solar power plant consisting of elongated modules whose longitudinal axes lie in a plane parallel to the direction of the sun rays to be converted. These modules are covered by lenses which refract sun rays impinging under an oblique angle downwards and which comprise two groups of refracting portions mirror-symmetrically arranged along the zenithal line which portions converge the refracted, downwardly directed sun beams into wedges of rays. These wedges of rays run twice per day in a vertical plane and during the remaining sunshine hours move within a narrow angular interval either towards the sun or away from the sun. This angular motion results in a vertical movement of the focal line so that the focal line coincides only twice per day with the height of the strip type photovoltaic cells. To force all wedges of rays to impinge evenly distributed onto the photovoltaic cell, a secondary lens is arranged above the photovoltaic cells. The light incident surface and shape of this lens produces the desired effect.

BACKGROUND OF THE INVENTION

Solar power plants floating on a body of water described so far have thedisadvantage that the photovoltaic cells have to follow the height ofthe focal line which changes with changing sun elevation.

SUMMARY OF THE INVENTION

The invention overcomes this disadvantage by a concentrator lens whichrefracts the noon rays into wedges of rays which are tilted in thedirection of the sun by about the same measure of angle as the wedges ofrays appertaining to low angles of incidence are tilted in the oppositedirection and by a secondary lens whose entrance surface is placed abovethe geometric locus of the focal point of a vertical wedge of rays andwhose exit surface is below the focal point of the wedge of rays withmaximum tilt.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows in a vertical cross section a part of a floating platform.

FIG. 2a, 2b, and 2c show the refraction of the insolation appertainingto different sun elevations.

FIG. 3a, 3b, and 3c show in a vertical cross section the paths of rayswithin the secondary lens.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION

FIG. 1 shows a cross section through part of a floating circularplatform which is carried by a thin layer of water 2. The platformconsists of concentrator tunnels 5 running parallel to each other whichare composed of elongated sheet metal profiles 1, optical concentratorpanels 3 and photovoltaic converters 4. The tunnels 5 are fastened to atoroidal tube 6 by a net of steel cords 8 and sheet metal bars 7 whichkeep the tunnels at a fixed distance from each other. The net isprevented from sagging flat pipes 9 which act also as down spouts forrain water. A trough 10 forming part of profile 1 runs along the middleline of each concentrator tunnel 5 and holds the photovoltaic cells 4 .The roof is formed by concentrator panels 3 which not only converge thesun rays by a first set of prisms but also deflect the originatingwedges of rays downwards by a second set of prisms. The photovoltaiccells 4 are arranged between bar-like secondary lenses 12 and the bottomof troughs 10. The toroidal tube 6, which is centered by rollers 13,rotates the platform around its vertical axis with the angular velocityof the sun, so that the photovoltaic cells follow the azimuth of thesun. During the night the platform moves back into the morning position.The secondary lenses 12 refract the wedges of rays independent of theposition of the respective focal line onto the photovoltaic cells 4. Avertically impinging wedge of rays 15 is shown within the concentratortunnel 5. Wedge of rays 15' however lies in an inclined plane. Wedge ofrays 15" shows the situation in which the wedge of rays has moved to theleft for instance due to a difference in heat expansion between thesteel cords 18 and the sheet metal profile 1. The gap between thetoroidal tube 6 and a wall 16 is bridged by a sliding film 17. Anotherfilm 18 covers the distance between the toroidal tube 6 and the adjacentconcentrator tunnel 5 so that no water will evaporate. The waste heat ofthe photovoltaic cells which is not reradiated during sunshine hourswill be stored in the water layer 2 which is separated from the subsoil19 by film 20. The heat stored by the water will be dissipated during 24hours daily by convection and infrared radiation.

The optical concentrator panels 3 are covered by a multiple oftransparent films, whereby the uppermost at the time can be peeled offif damaged.

FIG. 2 shows the function of the concentrating lens. FIG. 2a shows thatin the early morning and late afternoon hours the wedges of raysexperience the greatest refraction by the concentrator panels 3. Therefracted wedge of rays W₁ forms an angle of about -18° with a verticalplane. At this inclination the focal line F₁ has reached its highestlevel with the focal point B₁ lying on the side of the vertical planeaverted from the sun.

FIG. 2b shows that once in the morning and once in the afternoon at asun elevation of about 60° the wedges of rays W₂ run on a verticalplane. In this position the theoretic focal line F₂ lies far below thephotovoltaic cell 4 under the trough.

FIG. 2c shows that the sun rays at noon will be refracted in such a waythat the wedges of rays W₃ run within a plane which forms an angle of+18° with a vertical plane. At this angle of incidence the focal line F₃again reaches the same position as in FIG. 2a whereby the focal point B₃of said wedge of rays lies on the side of the vertical plane facing thesun.

FIG. 3 shows the path of rays within the secondary lens 12. The wedgesof rays W₁ and W₃ impinging on the secondary lens 12 under an angle ofabout ±18° to the vertical are refracted so that these wedges of raysare incident the photovoltaic cell 4 with an angle of ±11.9°. Inaddition to the refraction around an axis perpendicular to the extensionof the photovoltaic cell 4 a concentrating refraction takes placeparallel to the cells as in fresnel lenses. Since the focal distanceremains constant the focal line moves upwards with increasing angle ofthe wedge of rays with the normal. The photovoltaic cell 4 however has aconstant distance from the concentrator panels 3 thus the verticalmovement of the focal line F has to be adjusted by the secondary lens12. To attain this aim a geometry for the lens is chosen in which theentrance surface E of the secondary lens 12 has two convex areas E₁adjacent to the rims and a concave area E₂ in the center. The plane sidewalls S₁ and S₂ of the lens 12 converge towards the photovoltaic cell 4.

FIG. 3a shows that in the early morning, at noon, and late afternoon thefocal line F₁ appertaining to the lowest as well as to the steepest sunrays falls within the bar-shaped secondary lens 12. At the two extremesloping positions of the wedges of rays W₁ and W₃ the focal line F₁ hasits largest vertical distances from the geometric locus of the focalline F₂ appertaining to the vertical wedge of rays W₂.

FIG. 3b shows that between these two extremes the focal line F movesdownward and has its deepest position F₂ at a certain sun elevationbetween said extremes when the wedge of rays W₂ runs vertical. Due tothe curvature of the entrance surfaces E₁ and E₂ of the lens 12 andtotal reflection on the surfaces S₁ and S₂, all rays impinge on thephotovoltaic cell 4.

FIG. 3c demonstrates the capability of lens 12 to center even strayrays. Focal line F₁ ' is moved to the left, and focal line F₂ ' is movedto the right. All dislocated rays still impinge either directly or aftera total reflection on the planes S₁ and S₂ on the photovoltaic cell 4.

We claim:
 1. A solar power plant platform rotatable around its verticalaxis with concentrating means, which direct the sun rays by an opticalsystem onto energy converters, characterized in that the optical systemconsists of optical concentrating panels (3) covering the aperturesurface, having a first set of prisms extending horizontally,concentrating the sun rays (R) into wedges of rays (W₁, W₂ and W₃) and asecond set of prisms running perpendicular to said first set of prismsby which the wedges of rays (W₁) originated by sun rays (R₁) with a lowangle of incidence are refracted downwards forming an acute angle withthe vertical whereby their focal point (B₁) on the focal line (F₁) isaverted from the sun while the wedges of rays (W₃) originated by thesteepest sun rays (R₃) form a focal point (B₃) on the focal line (F₃)directed towards the sun and that the wedges of rays (W₁, W₂ and W₃) areguided to the energy converter through a lens (12), arranged above saidenergy converter, the lens having a light incident surface and shape torefract the vertically running wedges of rays (W₂) with a focal line(F₂) below said lens (12) and all other wedges of rays (W₁ and W₃) insuch a way that all rays impinge on the energy converter.
 2. A solarpower plant according to claim 1, characterized in that the energyconverter is a photovoltaic cell (4) and that its upper surface is indirect contact with the lower surface of the secondary lens (12).
 3. Asolar power plant according to claim 2, characterized in that thephotovoltaic cell (4) is in good heat conducting contact with the bottomof a trough (10) which rests on a layer of water (2).
 4. A solar powerplant according to claim 2, characterized in that the platform iscomposed of juxtaposed energy tunnels (5) consisting of a bottom portion(1, 10) of sheet metal comprising the photovoltaic cell (4), a roofportion comprising the optical panels (3) and vertically extendingmembers (9), connecting the bottom portion to the roof portion.
 5. Asolar power plant according to claim 4, characterized in that thevertically extending members (9) consist of tubes acting as down spoutsfor rain water.
 6. A solar power plant according to claim 1,characterized in that lens (12) forms a bar shape and that its entrancesurface consists of two convex strips (E₁) on both sides of a concavestrip (E₂).
 7. A solar power plant according to claim 6, characterizedin that the vertically extending walls (S₁ and S₂) of lens (12) aresymmetrical and converge towards the energy converter (4).
 8. A solarpower plant according to claim 1, characterized in that the wedges ofrays (W₁ and W₃) with extreme sloping position form almost the samemeasure of angle with the vertical but with different signs.
 9. A solarpower plant according to claim 1, characterized in that the opticalpanels (3) are covered with a multiple of transparent films, whereby theuppermost at the time can be removed if damaged.